Unable to boot into Ubuntu 14.10 Utopic Unicorn after upgrade

Diagnostics

After upgrading to Ubuntu 14.10 Utopic Unicorn, we can’t boot into Ubuntu anymore. On a dual boot system with both Ubuntu Linux and Windows 8.1 installed, the boot manager GRUB does not load. Instead, Windows is booted directly without any chance to select Linux.

After booting into the live system, follow the steps over here to chroot into your original Ubuntu installation and update-grub / grub-install grub from within there. Please note that the steps’ description on that page is in German. After a reboot, the system starts the grub boot manager again.

UEFI Secure Boot Specifics

On my system with an EFI installation of Ubtunu, I had to disable Secure Boot to be able to boot into Windows. With Secure Boot on, Windows failed showing a message similar to

/EndEntire
file path: (...)
error: cannot load image.

However, with Secure Boot turned off, Windows did not show up into grub after performing above steps.

Enabling Secure Boot,

running update-grub

and disabling Secure Boot again

made grub list Windows as boot option and Windows bootable from grub again.